The present invention relates to systems for detecting defects on a continuous, moving web of material and, more particularly, to detecting web defects in the form of streaks or scratches.
In the field of continuous sheet material manufacture and, in particular, paper manufacture, large, high speed machines manufacture and process paper in continuous webs. It is not unusual for commercial machines in mills to produce or coat paper that is 15 to 30 feet wide. The paper web itself moves at a rate of 3000 feet per minute during manufacture and up to 3500 feet per minute during coating.
The processing of paper on these continuous, industrial machines requires cutting blades, instruments and fixtures that may damage the paper itself. Paper is, of course, a delicate substance by nature, but high speed machines are made of metal and other non-resilient materials. Added to the inherent fragility of the paper may be coated surfaces for photographic, radiographic, xerographic or other uses, which coating or emulsion is even more delicate. Moreover, the coating process itself relies on a coating hopper feed slot or coating bead, which is susceptible to entraining air and entrapping it in the work product itself, affecting the coating thickness.
It is possible and even likely that defects and irregularities attributable to misaligned machine components will become manifest on the paper product itself. Not only may blades be defective, but contamination or particulate matter may become trapped between the blade and the working surface. Even such seemingly innocuous elements as oversized silver grains or gelatin slugs may contribute to undesired production of so-called pencil streaks.
When a machine element does contact the paper, the result is usually not merely an isolated defect, but a streak or scratch that runs along the entire, endless length of the web. Such occasional events are not entirely preventable, as a practical matter, and must be corrected immediately upon detection. Due to the speed of the machinery, each second from detection to correction results in 50 feet of defective paper product.
Unfortunately, defects such as streaks and scratches that run along the major axis of web material have been difficult or impossible to detect during the processing thereof. The problem of streaks and scratches has received considerable attention, of course, but a consistent, highly accurate approach to detecting such defects has not been found. While certain technology has permitted the detection of isolated defects, such as dust particles in a coated layer or air bubbles causing dry spots or holes, subtle scratches have eluded detection.
In U.S. Pat. No. 4,779,005 issued to Jack L. Arnold, a focal plane electronic system is disclosed, in which a two-dimensional array of photo-detectors receives pixel information from a scanned scene. Each pixel is observed by a plurality of detectors, and their signals are summed by time delay and integration circuitry. The system is included in a Z-type, three-dimensional focal plane module. High frequency noise due to spots and other localized anomalies, however, cannot be eliminated.
U.S. Pat. No. 4,274,748 issued to Jean Burtin et al teaches a system by which streaklike defects in running webs and sheets are identified by transversely and repeatedly scanning adjacent lateral sections of the webs or sheets by means of sweeping radiant energy, such as a moving radiation spot. Thus the scanner is adapted to move perpendicular to the direction of the web motion. Transmitted or reflected radiation is received on corresponding photocells that produce a plurality of discrete, successive measurement signals representative of a defect in the webs or sheets. The signal-to-noise ratio of the measurement signals is increased by multiplicative correlation, on the condition that the ratio of the signal component to the effective noise component of the measurement signal is greater than one.
U.S. Pat. No. 3,754,146 issued to Allan TitShing Chow discloses a detecting head and light source which synchronously traverse on opposite sides of a continuous moving web. Photoelectric detector elements are recessed in rectangular channels disposed symmetrically about rectangular coordinate axes. The principal axis of the rectangular channels is in the direction of web travel. A streak defect in a coated layer on the web interferes with the transmitted light to the detecting elements, resulting in an electrical characteristic signal across the detecting elements. The signal is processed for verification of the presence of a streak defect.
The present invention exploits the use of video technology to solve defect problems. It is interesting to note that none of the foregoing references incorporates a video camera for detecting streaks or scratches on a moving web, relying instead on one or more photodetectors to accomplish that purpose. Discrete photocells are used instead. In the case of the aforementioned Arnold patent at least, charge coupled devices (CCD's) are specifically excluded due to their alleged inefficiency.
One of the difficulties in using video images, and perhaps the one to which Arnold alludes, is processing data by means of a technique called "frame grabbing," in which an entire video image is analyzed, pixel by pixel. Although this technique is used extensively in satellite photography, its use in real time paper flow manufacture and processing is limited. This technology is simply too slow to address the machine speed verses resolution problems in commercial paper manufacture. More generally, computerized imaging is inherently a batch process, not suitable for real time continuous processing of paper. Frame grabbing technology is also memory intensive, since each frame contains hundreds of thousands of pixel values which must be processed by computer while keeping up with the machine. An alternative has been to statistically inspect the web material, but such an approach is not acceptable to manufacturers who guarantee uniform, defect-free products. In the case of photographic paper, for example, stringent quality standards specify that even the most subtle streaks cannot be tolerated.
It would be advantageous to provide a high-speed, real time system for detecting defects and irregularities, such as streaks and scratches, in moving webs of material.
It would also be advantageous to detect streaks and scratches on a high speed paper coating or paper production machine.
It would be also advantageous to inspect 100% of processed paper for such defects as a part of quality assurance and control program.
It would also be advantageous to inspect moving webs or material continuously, rather than statistically or repeatedly.
It would also be advantageous to incorporate the use of area scan cameras to detect such defects.
It would also be advantageous to detect streaks and scratches on or in a web of material by using a technique of specular energy reflection.
It would also be advantageous to provide a system for processing video signals to result in a meaningful, suitable display.